Consumer goods such as electrical products, furnitures, building materials, auto parts, are being reassessed as to the steel sheet used in their external structures from the viewpoint of recycling, elimination of harmful substances in waste disposal, and the ecology. Further, even among the steel sheets used, ferrous metal manufacturers are painting the outer surface of the steel sheet with the paint of the final product and using this as starting material. Therefore, the precoated steel sheet is being viewed with interest as being capable of eliminating the painting step at the manufacturer of the final product.
The problem with using such a precoated steel sheet as an outer plate is that, when cutting the precoated steel sheet to the final dimensions of the finished product, the cut edge becomes a bare ferrous metal having no paint or plating. Therefore, for products which are to be used in environments requiring rust-proofing (even products used in normal indoor environments often require rust-proofing at portions exposed at the outside in view of the image of the product) and products where the strange feeling given by the color of the paint of the precoated steel sheet and the metallic luster of the cut edge would detract from the image of the product, one either has to give up on the use of a precoated steel sheet (use an ordinary steel sheet and paint it after processing) or else paint the cut edge after the processing.
Note that in the case of such post-painting of the cut edge, it is necessary to prevent the paint from spreading to the decorative surface of the precoated steel sheet so as to protect the beauty of the decorative surface. However, the thickness of the steel sheet used here is about 1 mm or an extremely thin steel sheet of even less than 1 mm. Painting such a thickness of a cut edge without spreading over to the decorative surface is extremely difficult even using various manual painting methods such as spray painting or roller painting--it is close to impossible to perform by automatic painting machines.
In the past, the technique known as "masking" is used to deal with this problem. For example, as shown in FIG. 1, the portion on the precoated steel sheet 1, on which spreading of paint was desired to be prevented, was covered in advance by masking tape 2 along the surface to be painted, the painting was conducted in that state, then the tape was peeled off after painting. Thus the beautiful painting free from spreading to the surface to be protected can be obtained. By masking in this way, various methods of painting were possible. However, when attaching the masking tape, it is necessary to attach the tape exactly along the complicated contour of the processed surface of the product. This could only be done by careful manual work by workers and was therefore a costly, time-consuming technique.
As a prior art relating to coating of a chemical on to the end edge of a plate material, while differing in the field of art, Japanese Unexamined Patent Publication (Kokai) No. 5-263271 has proposed a technique of coating a photoresist on to the end edge of a lead frame plate.
However, in the protective painting of the cut edge of a precoated steel sheet to which the present invention is concerned, it is most preferred from the perspectives of the cost of the painting and the beauty of the exterior appearance that only the cut edge and its surroundings be painted, and therefore, the spray painting of the above prior art is not suitable for the protective painting of the cut edge of a precoated steel sheet even assuming masking.
Further, in the case of simple roller painting, the cut edge of the steel sheet near the knife edge, where in general even burrs remain, is painted, and therefore, there is the problem of easy damage to the roller. On top of this, the portion of contact becomes the point contact of the painting roller and the cut edge of the steel sheet and therefore, the actual contact area becomes small. Accordingly, a large fluctuation in the contact force of the painted portion per unit area unavoidably occurs. In particular, in the case of thick painting providing a thickness of several tens of microns so as to cover burrs on the cut edge, it is difficult to paint uniformly and stably.
Further, the prior art of coating a chemical on to the end edge of a plate material disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-263271 describes an invention for coating a resist on the end edge of a lead frame plate. Not only does the field of art differ, but when making a painting device held by, for example, a robot and using the motion of the robot to perform the painting as in the present invention and, when the portion to be painted is an open end edge, the application thereof becomes difficult. Accordingly, improvement of the method and apparatus of painting and rust-proofing the cut edge or end edge of a precoated steel sheet has been desired.
Further, the decisive point in using a precoated steel sheet as an exterior plate is that shaping giving a large deformation easily causes damage to the coating of the precoated steel sheet at the portion coming into contact with the shaping die, when shaping the original precoated steel sheet. Therefore, when shaping a product to a large extent, the practice has been to attach to one or both surfaces of the original precoated steel sheet a plastic film serving as both a lubricating film and protective film at the time of shaping by the die.
The plastic film which is attached, however, has to be removed after the shaping or after the assembly of the product (almost always manually peeled off by workers). Considering the work efficiency at this time, it is not possible to adhere the plastic film too strongly to the precoated steel sheet.
In recent years, the quality of steel sheet and coatings have been improved and it has become possible to perform the shaping of the steel sheet even with major deformation, but in products making use of this feature, in particular when performing sophisticated shaping in several steps, if performing the first shaping, then, for example, peeling off the adhered plastic film, as partially shown in FIG. 20 and FIG. 25 explained below, and performing the next shaping step in that state, the plastic film portion wrinkles and is printed on the product and thereby becomes a cause detracting from the appearance of the product and a cause of flaws in the expensive die. Accordingly, improvements have been sought in the method for attaching a plastic film to a precoated steel sheet capable of preventing flaws in the painted surface at the time of shaping a product of a structure using a precoated steel sheet for the exterior plate.